A variety of approaches are used to reduce energy consumption in commercial or residential buildings, as well as in the automotive industry to help maintain a comfortable temperature in the passenger cabin with minimum energy expenditure. For example, dyed and vacuum-coated plastic films have been applied to windows to reduce heat load due to sunlight. Typically, heat load reduction is accomplished by blocking solar radiation in the visible or the infrared portions of the solar spectrum, or both (i.e., at wavelengths ranging from 400 nm to 2500 nm or greater).
In general, dyed films can control the transmission of visible light, primarily through absorption, and consequently also provide glare reduction. However, dyed films generally do not block near-infrared solar energy and are not completely effective as solar control films. Dyed films also often fade with solar exposure. In addition, when films are colored with multiple dyes, the dyes may fade at different rates, causing unwanted color changes over the life of the film.
Other known window films for solar control include those fabricated using vacuum-deposited grey metals (e.g., stainless steel, inconel, monel, chrome, and nichrome alloys, among others). The deposited grey metal films offer about the same degree of transmission in the visible and infrared portions of the solar spectrum. As a result, in general, the grey metal films are an improvement over dyed films with regard to solar control. The grey metal films are relatively stable when exposed to light, oxygen, or moisture, and in those cases in which the transmission of the coatings increases due to oxidation, color changes may not be generally detectable. After application to clear glass, grey metals block light transmission by approximately equal amounts of solar reflection and absorption. While the use of grey metal for solar control window films is quite common, metals such as copper or sometimes even ceramic materials such as titanium nitride is used.
Other window films for solar control include those with vacuum-deposited layers of certain metals, such as silver, aluminum, and copper, which control solar radiation primarily by reflection. Certain thin metal films may remain semi-transparent in the visible spectrum and reflect near infrared radiation are used in solar control glazing applications. Most often, silver or silver alloys is the choice of metal due to its high reflectance in the infrared region. However, window films having a metal layer of a sufficient thicknesses to achieve a high level of near infrared reflection may also have significant reflection in the visible region. Under certain circumstances, the reflection of thin metal layers in the visible region could be suppressed by cladding the silver layer on both sides with a high refractive index dielectric layer having a metal oxide with a sufficiently large thickness. The combination of refractive index and thickness (optical thickness) of these layers can be chosen in order to achieve high visible transmission and low reflection.
There is a continuing need for high visible light transmission (e.g., >70%) and low emissivity (e.g., less than 0.2) films. The present disclosure describes novel durable low emissivity films that could be used as solar control films, and which have low visible reflectance and high visible transmission.